Prestressed concrete structure and method of making the same



Nov. 26, 1963 R. w. HODGE 3,111,965

PRESTRESSED CONCRETE STRUCTURE AND METHOD OF MAKING THE SAME Filed Feb.21. 1958 E I I! 'HWW INVEVTOR.

2 0162; Haifa.

zw wm United States Patent 3,111,965 PRESTRESEED CONCRETE STRUCTURE ANDMETHUD OF MAKING THE SAME Robert W. Hodge, Lake Angelus, Mich. (1307Pontiac State Bank Bldg, Pontiac 15, Mich.) Filed Feb. 21, 1958, Ser.No. 716,637 3 (Claims. (til. 138-l76) This invention relates toprestressed concrete articles and to a method of making them. Moreparticularly, the invention relates to the prestressing of generallycylindrical or annular concrete structures such as storage tanks,pressure containers, sections of pipe and the like.

The prestressing of concrete structures may be accomplished by applyinga longitudinal stress or tension to one or more wires or cables, andthen casting concrete around the stressed wires and allowing theconcrete to set to obtain a bond between the concrete and theprestressing elements. After a firm bond has been obtained, theprestressing elements exert a compressive force on the concrete in thedirection or" the length of the elements to give very great beamstrength and shear resistance to the concrete structure. When theprestressed structure is of generally cylindrical or annular form, it isfrequently desirable or necessary that the article he prestressedcircumferentially as well as longitudinally. In the past thelongitudinal and circumferential prestress has been accomplished in twoseparate steps or operations and generally consists in casting theconcrete around longitudinal prestressing elements and, after hardeningof the partially formed structure, winding circumferential prestressingelements around the structure and casting an additional shell orcovering over the circumferential prestressing elements.

The present invention contemplates a longitudinally andcircumferentially prestressed concrete structure of generallycylindrical form in which each prestressing element provides bothlongitudinal and circumferential prestressing of the article, therebymaking it possible to fabricate such structures in a single casting orother forming operation. This objective is accomplished by employingprestressing elements each of which corresponds to one of thestraight-line elements of a hyperboloid of revolution, with theangularity of the successive elements in the series being oppositelydirected to eliminate torsional stress and provide a stable structure.

A principal object of the invention is to provide a new and improvedprestressed concrete structure.

A further object of the invention is to provide a new and improvedmethod of making prestressed concrete articles.

Another object of the invention is to provide a means for obtaining bothlongitudinal and circumferential prestress in a generally cylindricalconcrete article by the use of a single series of prestress elements.

Other and further objects of the invention will be a parent from thefollowing description and claims and may be understood by reference tothe accompanying drawing, which by way of illustration shows a preferredembodiment of the invention and what I now consider to be the best modein which I have contemplated applying the principles of my invention.Other embodiments of the invention may be used without departing fromthe scope of the present invention as set forth in the appended claims.

In the drawing:

FIG. 1 is a perspective view with parts broken away illustrating thearrangement of the prestressing elements in a form or mold in which acylindrical structure is to be cast;

FIG. 2 is a fragmentary perspective view of a section a 3,1 l L965Patented Nov. 26, 1963 ice 2 of cylindrical pipe made according to thepresent invention;

FIG. 3 is a diagrammatic view illustrating certain principles of theinvention; and

FIG. 4 is an elevational view of a series of prestressing elementsarranged in accordance with the present invention and illustrating afurther modification of the invention.

As stated above, the present invention is applicable to the productionof hollow concrete structures of various types having generallycylindrical or ellipsoidal cross sections and is particularly suited forthe production of concrete pipe, pressure containers or storage tanksand similar concrete structures. The invention is illustrated in thedrawing as applied to the production of a section of cylindricalconcrete pipe indicated at it) in FIG. 2. The apparatus employed forcarrying out the invention may be varied as required according to thetype of structure being reduced, and the apparatus disclosed in FIG. 1is merely illustrative of one form of apparatus that may be used.

in FlG. 1 there is disclosed a concrete form or mold comprising an innercore 12 and an outer shell 14 spaced apart a distance corresponding tothe wall thickness required in the finished product. A pair of endplates 16 and 18 are suitably supported on either the inner or outerform 12 or 14 and, together with the forms 12 and 14, define a spaceconforming to the shape of the article to be produced.

The end plates 16 and 18 are provided with suitable means to receive andclamp the ends of a series of prestressing elements in the form of wiresor cables 2d. Each of the prestressing elements 2% corresponds to one ofthe straight-line elements of a hyperboloid of revolution having itsaxis coincident with the axis of the concrete article to be produced. Asshown in FIGS. 1 and 4, certain of the elements 29 extend from the upperend plate 18 to a point on the lower end plate 16 which is advancedapproximately 30 in a counterclockwise direction from the point on thelower end plate is which corresponds to the point at which such elementsare fixed to the upper end plate 18. The remaining prestressing elements2i) have an opposite angularity to provide a cage-like structure ofannular form.

After the prestress elements 2.6 have been arranged in the mannerdescribed, one of the end plates 15 or 18 may be moved axially away fromthe other to impart a longitudinal stress or tension to the elements 29.Any suitable means may be employed for stressing the elements 25 such ashydraulic jacks, screw jacks or the like, the particular apparatus usednot being illustrated herein. It is contemplated that the wires 29 maybe stressed individually or that a series or all of the elements may bestressed at the same time. After the elements 2%} have been uniformlystressed the required amount, concrete or other cementitious material ispoured or otherwise forced into the mold in any suitable manner andallowed to harden while the tension is maintained on the prestressingelements Zil. The length of time required for bonding the concrete tothe prestressing elements will depend upon the type of material used,but in any event should be long enough to obtain a firm bond between theconcrete material and the prestressing elements. The tension on theelements may then be released and the article removed from the mold.Adjacent wires Zn in the series have opposite angularity, so thatone-half of the wires correspond to the first generation elements of thehyperboloid of revolution While the remainder correspond to the elementsof the second generation. An article produced in accordance with theabove described method will have both a longitudinal and acircumferential pre- 3 stress, as will be describedin connection withthe force diagram of FIG. 3.

In FIG. 3 the two circles represent the upper and lower circles of thehyperboloid of revolution which is approximated by the entire series ofelements 20. The line H is perpendicular to both circles and representsthe height or length of the article to be produced, R indicates theradius of either circle, and angle A is the angle through which thelower end of an element 20 is advanced in a counterclockwise directionfrom the position of the upper end of the element 20 on the uppercircle, from which the length of chord C can readily be determined. Fromthe foregoing it can be calculated that the angle B that the element 20makes with the line H can be determined by the equation:

2R sing Having determined the angle 13, and with a tension of F poundson the element 29, it will be seen that the tension on the element willbe resolved into longitudinal prestress of F cos B, and acircumferential prestress of F sin B.

By varying the angle A, the distribution of the prestress in thelongitudinal and circumferential directions may be varied as desired. Itis apparent that, as the angle A is increased, the wall thickness of thearticle must be increased. While in the illustrated embodiment of theinvention a cylindrical article has been shown, it will be apparent thatarticles having a larger diameter at one end than at the other may beproduced according to the present invention. The invention is alsouseful in the production of articles ranging from relatively smalldiameter pipe up to very large structures such as tanks or the like, andsimilarly the prestressing elements may range in size from smalldiameter wires up to large diameter steel cables.

In FIG. 4 there is disclosed an arrangement of prestressing elements 20arranged as described above and having a longitudinal stress appliedthereto. In order to provide additional reinforcement circumferentiallyof the structure, a series of additional wires or cables 22 may bewrapped around and suitably fastened to the prestressing elements 22 Theadditional reinforcing elements 22 need not be prestressed or tensionedbut would perform the function of taking up or resisting the load on theprestressed concrete structure when the bursting strength of thepretensioned wall is reached. Such additional circumferentialreinforcing elements would materially increase the pressure capacity ofthe article.

Tan B= From the foregoing it will be seen that the present inventionprovides a relatively simple and inexpensive means of making articlesof. the type described having both longitudinal and circumferentialprestress by a series of similarly arran ed prestressing elements.

While I have illustrated and described a preferred embodiment of myinvention, it is understood that this is capable of modification, and Itherefore do not wish to be limited to the precise details set forth butdesire to avail myself of such changes and alterations as fall withi inthe purview of the following claims.

I claim: l

1. The method of making longitudinally and circumferentially prestressedconcrete structures of generally annular cross-section in a singlecasting operation, which comprises arranging a series of wires to form acage-like structure with each wire conforming substantially to astraight line element of a hyperboloid of revolution and with successivewires in the series having equal and opposite angularity, tensioningsaid wires longitudinally thereof while maintaining said wires in saidstraight line condition, casting cementitious material around and oversaid tensioned wires in the form of the structure to be produced, andallowing said material to harden while maintaining said tension to forma bond between said material and said tensioned wires whereby each ofsaid wires imparts both longitudinal and circumferential prcstress tothe hardened material.

2. A prestressed concrete structure of generally annular cross-sectionhaving embedded therein and bonded thereto a series of longitudinallytensioned prestressing wires each of which corresponds to a straightline element of a hyperboloid of revolution having its axis coincidentwith the axis of said structure, successive wires in said series havingequal and opposite angularity whereby the series of wires imparts bothlongitudinal and circumferential prestress to said structure.

3. A structure according to claim 2 including a series ofcircumferentially extending, longitudinally spaced reinforcing wiressurrounding said prestressing wires and also embedded in and bonded tosaid concrete.

References Cited in the file of this patent UNITED STATES PATENTS950,264 Hugo Feb. 22, 1910 958,043 Stockley May 17, 1910 2,522,165 CrornSept. 12, 1950 FOREIGN PATENTS 457,227 Italy May 12, 1950

2. A PRESTRESSED CONCRETE STRUCTURE OF GENERALLY ANNULAR CROSS-SECTIONHAVING EMBEDDED THEREIN AND BONDED THERETO A SERIES OF LONGITUDINALLYTENSIONED PRESTRESSING WIRES EACH OF WHICH CORRESPONDS TO A STRAIGHTLINE ELEMENT OF A HYPERBOLOID OF REVOLUTION HAVING ITS AXIS COINCIDENTWITH THE AXIS OF SAID STRUCTURE, SUCCESSIVE WIRES IN SAID SERIES HAVINGEQUAL AND OPPOSITE ANGULARITY WHEREBY THE SERIES OF WIRES IMPARTS BOTHLONGITUDINAL AND CIRCUMFERENTIAL PRESTRESS TO SAID STRUCTURE.